The improved tension spring can be used in place of the known tension spring, which depending on the executions is mounted on the bobbin case in case of a central bobbin hook, and on the bobbin case or directly on the basket in case of a rotary hook.
The rotary hook can be of the type with horizontal axis of rotation or of the type with vertical axis of rotation.
Lockstitch sewing machines and related central bobbin hooks or rotary hooks are well known and therefore will not be described here, limiting just to remember schematically their composition.
The central bobbin hook comprises a hook body on which is bound the bobbin case that contains the bobbin on which the bobbin thread is wound. The bobbin thread is threaded through the guiding holes of the bobbin case and is made to pass between the tension spring mounted on the bobbin case and the bobbin case itself, so as to create a tension on the thread during the unwinding of the same for the sewing operation. This tension can be adjusted by turning the adjusting screw of the tension spring.
Quite similar is the operation of the bobbin case—bobbin system in the rotary hooks, where the tension spring is always mounted on the bobbin case to create the tension of the bobbin thread, similarly as described for the central bobbin hook. The difference though is that on the rotary hooks, in execution with bobbin case, the bobbin case is constrained to the basket housed inside the hook body, instead directly to the hook body.
On the rotary hooks in execution without bobbin case, on the other hand, the bobbin is housed directly in the basket and the tension spring that creates the tension on the bobbin thread, is mounted on the basket and the bobbin thread is threaded through the guiding holes of the basket and is made to pass between the tension spring mounted on the basket and the basket itself so as to create a tension on the thread during the unwinding for the sewing operation. This tension can be adjusted by turning the adjusting screw of the tension spring.
Beyond where the tension spring is mounted (on the bobbin case or on the basket) its operating principle is always the same: the tension spring is formed by a thin metal sheet of a thickness generally between 0.2 mm and 0.5 mm, of elongated shape with a major axis a few times longer than the minor axis and bent appropriately at arch along its major axis, so as to press the bobbin thread on the wall of the support means on which the tension spring itself is mounted (respectively the bobbin case or the basket), thus creating a friction on the bobbin thread, which is then put under tension during unwinding for the sewing operation. The pressure with which the tension spring presses on the bobbin thread can be adjusted through an adjusting screw. Acting on this adjusting screw and varying the pressure, the bobbin thread tension is varied for the sewing. The tension spring is mounted on its support means (respectively the bobbin case or the basket) by means of said adjusting screw and a fastening screw or, alternatively to said fastening screw, by an interlocking system on the support means. Important for the thread tension's stability is the contact point between the tension spring and the bobbin thread. In the standard state of the art execution, this contact point is located on the edge of the tension spring. In absence of the bobbin thread it is in fact natural that the arch shaped bend of the tension spring leads said tension spring to rest on the support means, on which it is mounted, along the extremity of the tension spring that is on the edge of the sheet that constitutes it. In a second implementation (actually very rare), instead, the contact point is set back with respect to this edge and is determined by the point of tangency of the arc of bending of the tension spring and the profile of the support means on which the tension spring is mounted. In this case, however, it is much more difficult to identify with certainty the contact point, and it is much more difficult to keep constant the repeatability of such assembly during the mass production of the tension spring, which is always a thin bent metal sheet and as such presents discrete machining tolerances.
The first case, namely that of the pressure point corresponding to the edge of the tension spring's sheet is universally used and has the advantage of an easy realization. A first drawback, however, is constituted by the tension of the bobbin thread that is not stable (i.e. has oscillations) during the sewing operation as it is very dependent to the irregularities of the thread itself: pulling the thread by hand gives the feeling that the tension spring “scratches” the thread. This effect is much more evident with certain types of threads, such as those more rough and those of poor quality and is much more annoying when the sewing requires a low tension of the bobbin thread, as it causes irregularities in the closure of the stitch. A second disadvantage of this execution occurs when, at the change of the type of bobbin thread, also the tension generated by the pressure of the tension spring changes. Consequently, at each change of bobbin thread type is necessary to re-adjust the pressure, acting on the adjustment screw. This effect is obviously more inconvenient in applications that need a frequent change of the bobbin thread type, as occurs in household and handicraft activities.
The second case, namely that of the pressure point corresponding to the point of tangency of the arc of bending of the tension spring and the profile of the support means on which it is mounted, in part fixes the first mentioned disadvantage, as it reduces the fluctuations of the tension and reduces the tension's sensitivity to the irregularities and roughness of the bobbin thread. However, changing the diameter of the bobbin thread, also the pressure point can change, thus changing the tension. The main disadvantage of this second case, however, is given by the difficulty of mass production of such a tension spring and by a poor repeatability from one tension spring to another. These factors generate a much higher cost of production, in addition to a remaining intrinsic uncertainty concerning the exact point of pressure.
The U.S. Pat. Nos. 6,152,057 and 6,901,871 patents disclose a tension system in which the bobbin thread is wrapped at least partially around the element that generates the tension. The tension of the bobbin thread is therefore obtained by friction of the bobbin thread on the element that generates the tension.
The U.S. Pat. No. 6,895,879 patent describes a tension spring with a different section between the portion on which acts the adjustment screw and the portion on which the tension spring presses the thread. The purpose is to have a greater flexibility of the portion on which the tension spring presses the thread.
The CH-A-658 273 patent refers to a shuttle for textile machines and embroidery machines comprising a cover on which is fixed a flat spring arranged parallel on the cover and apt to give tension to the bobbin thread.